Saying just "it doesn't work" is useless. What exactly doesn't work? What error(s) do you get? I did a test before posting my comment and it worked, so please explain what exactly doesn't work.
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Gonzalo MedinaDec 7 '11 at 19:44

@GonzaloMedina It compiles but I see both the images. I now also add something in the preamble. Maybe the dynamic option is the problem?
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BaduelDec 7 '11 at 19:47

@Baduel I don't think it is possible with \includegraphics, as beamer does not know how to make the image semitransparent. That is in fact why you saw both images in your first version. You can try drawing a tikz picture instead of using \includegraphics, and it should show semitransparent even in your first code version.
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silvadoDec 8 '11 at 11:49

Ok thanks. I don't know tikz, but I will remedy.
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BaduelDec 8 '11 at 11:56

Just make only the second one only. Also note that you can use ranges, e.g. only<2-> for one image and only<3-> fo the next. Then in the first subslide no image would be seen, in the second only the first, and in the third (and all following, if any follow) both.
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celtschkDec 7 '11 at 20:06

@GonzaloMedina Very nearly to what I want. Is it possible to preserve the absolute position in each step?
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BaduelDec 7 '11 at 20:42

Also note that want to do this in order to print the frame in only one slide, when I will put the handouts option.
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BaduelDec 7 '11 at 20:50